1. Field of the Invention
The present invention relates to a composition for forming a piezoelectric film, a method for producing a piezoelectric film, and an ink jet recording head provided with such piezoelectric element.
2. Related Background Art
A piezoelectric element is constituted of piezoelectric crystalline ferroelectric or paraelectric ceramics. The piezoelectric ceramics generally have a two-component system principally constituted of lead zirconate-titanate (hereinafter called “PZT”), or a three-component system in which a third component is added to PZT of the two-component system. A ferroelectric substance employing the two-component PZT is described in Applied Physics Letters 1991, vol. 58, No. 11, pp 1161–1163. For forming a piezoelectric thin film of such metal oxide type, there are known a sputtering method, an MOCVD method and a sol-gel method.
There is also disclosed an ink jet printing head utilizing a piezoelectric element employing a film formed by the sol-gel method. For example, Japanese Patent Applications Laid-open Nos. H9-92897, H10-139594 and H10-290035 disclose a method forming a piezoelectric thin film of a piezoelectric element usable for an ink jet printing head, by the sol-gel method, by coating a sol containing a piezoelectric material plural times on a lower electrode and repeating a heating process.
However, by simply dissolving an ordinary metal complex or a metal salt of an organic acid in a solvent as a composition for forming a piezoelectric film by the sol-gel method, it is difficult to obtain a uniform composition since the organometallic compounds having respectively different hydrolyzing rates respectively form single metal oxides. Also at sintering process, a complex is formed by a solid-phase reaction of such metal oxides, but it is difficult to obtain a uniformly controlled composition in the thin film because of a difference in the volatility thereof. Also at the film formation, there is often formed a film of a fine powder state, tending to generate an electrical conductivity.
Against these issues it is known to add a stabilizer for controlling a rapid hydrolysis, and such method is considered to contribute to the stabilization of the coating liquid and the stabilization of the film formation.
As the stabilizer, there have been utilized a β-diketone, a ketoacid, a lower alkyl ester of such ketoacid, an oxyacid, a lower alkyl ester of such oxyacid, an oxyketone, an α-amino acid, an alkanolamine etc. Such stabilizer stabilizes a metal alkoxide and/or a metal salt by chelating, thereby reducing the rate of hydrolysis reaction. For example an alkoxide of Ti or Zr is reacted with acetylacetone to reduce the reaction rate of hydrolysis, thereby obtaining a dense film.
However, the use of such stabilizer may still be unable to provide a satisfactory result because a conversion from an alkoxide to an oxide is incomplete to generate an incompletely dense film composed of fine particles, thereby leading to a fluctuation in the composition by a loss of a more volatile metal component or to a fluctuation in the piezoelectric property. Also a film formation with the above-mentioned stabilizer result in uneven sizes of the particles constituting the thin film. In the thin film with such uneven surface state, the piezoelectric property may become uneven depending on the location.
Also from the standpoint of the manufacture of a piezoelectric element, it is considered more efficient to obtain a larger film thickness in a single layer to be formed, and necessary to have measures for attaining such thickness. Also for use as an actuator, there is required a dense film showing uniform characteristics as a piezoelectric element on the film surface and being excellent in the durability. Also a formation of a thick film results in defects such as a cracking in the film, and is associated with insufficient characteristics as an actuator. Also a conventional method for producing a composition for forming the piezoelectric film, the composition may show a precipitate generation by a polymerization reaction in the liquid in case of a prolonged storage, thus not being usable in stable manner over a prolonged period.